The present invention relates to an in-line electron gun for a color picture tube and, more particularly, to a main lens structure suitably adapted for improving the focusing characteristics.
In the in-line electron gun, the main lens electrodes for the respective electron beams must be aligned laterally in a row, and the diameter of each main lens thus becomes smaller than that for a delta electron gun. The diameter of the neck cannot be increased beyond a predetermined value due to limitations imposed by deflecting power, convergence of three electron beams and so on. Furthermore, the gap between the outer diameter of the electron gun and the inner diameter of the neck must be over, for example, 1 mm so as to prevent deterioration of the glass inner wall of the neck due to irradiation with electrons. Accordingly, the diameter of the main lens cannot be increased as desired.
FIG. 1 is a plan view of an electrode for forming the main lenses of a conventional electron gun as disclosed in Japanese utility model publication No. 51648/81, and FIG. 2 is a partially broken front view thereof. Referring to FIGS. 1 and 2, a cup-shaped electrode 1 has three cylindrical portions 2 for passing three electron beams therethrough aligned laterally in a row along the x--x direction. Two such electrodes 1 are combined such that the respective cylindrical portions 2 face each other at surfaces 3 so as to form three main lenses for respective three electron beams. In such an electrode, since a bridge portion 4 between each pair of adjacent cylindrical portions 2 does not play an essentially important role in forming the electron lens, it is preferable that the bridge portion have as small a size as possible. However, reduction in the size of the bridge portion 4 is limited by tolerance of a pressing die. For this reason, the opening or inner diameter of each cylindrical portion 2 becomes about 80% of a pitch p between adjacent cylindrical portions 2. If the electrode is to be housed within a neck having an outer diameter of, for example, 29 mm, the inner diameter of each cylindrical portion 2 becomes 5.5 mm for a pitch p of 6.6 mm. For information, each cylindrical portion of a delta electron gun will have an inner diameter of 6.35 mm under the same conditions.
When the opening of each cylindrical portion of the electrode is small, the focusing performance of the main lenses is degraded, resulting in poor resolution.